1. Field of the Invention
The present invention generally relates to an interference canceller, and more particularly to an interference canceller suitable for a cellular DS/CDMA (Direct Sequence Code Division Multiple Access) mobile communication system or the like.
In a cellular CD/CDMA mobile communication system, an interference occurs which results from an interference and noise from another mobile station due to correlation between spread codes caused by asynchronism with mobile stations. Such an interference serves as a factor which degrades the channel capacity and the transmission quality of the mobile communication system. Hence, it is desired to precisely eliminate such an interference from a received signal.
2. Description of the Related Art
FIG. 1 shows a conventional multistage type interference canceller. Each of the stages in the multistage type interference canceller is made up of interference canceller units 81 and a combiner 82. The stages thus configured are cascaded. FIG. 1 shows the multistage type interference canceller having the first stage through the mth stage. Data symbol receivers 83 are provided in the mth stage, which is the final stage.
The interference canceller units 81 and the final stage are provided in parallel for the respective users"" channels. The suffix of the reference number 81 indicating the interference canceller units 81 includes a stage number and a user number corresponding to the user channel (ICU1,1, ICU1,k, ICU2,1, ICU2,k . . . ).
In the first stage, a received signal R0 is input to the interference canceller units ICU1,1-ICU1,k corresponding to the users"" channels, which output interface replica signals S1,1-S1,k and interference residual signals d1,1-d1,k. The combiner 82 combines the interference residual signals d1,1-d1,k corresponding to the users"" channels. The combined interference residual signals d1,1-d1,k are subtracted from the received signal R0, so that a resultant error signal e1 of the first stage is obtained.
In the second stage, the interference canceller units ICU2,1-ICU2,k are supplied with the error signal e1 from the combiner 82 of the first stage and the interference replica signals S1,l-S1,k from the interference canceller units ICU1,1-ICU1,k of the first stage. Then, the interference canceller units ICU2,1-ICU2,k respectively output interference replica signals S2,1-S2,k and interference residual signals d2,1-d2,k. The combiner 82 combines the interference residual signals d2,1-d2,k corresponding to the users"" channels. The combined interference residual signals d2,1-d2,k are subtracted from the error signal e1 of the first stage. Hence, an error signal e2 of the second stage is obtained.
In the mth stage, which is the final stage, the receivers ReCm,1-ReCm,k are supplied with an error signal em-1 and interference replica signals Sm-1,1-Sm-1,k of the previous stage, and perform an interference eliminating process using the supplied signals, so that data symbols can be decoded. By sequentially repeating the interference eliminating process, the error signal is gradually reduced, and interference replica signals can be obtained from which signals interference between the users can be eliminated.
FIG. 2 shows a conventional interference canceller unit, which includes despread processing parts 91, a despreader 91-1, an adder 91-2, a multiplier 91-3, a channel estimation circuit 91-4, a combiner 92, a decision part 93, spread processing parts 94, a multiplier 94-1, an adder 94-2, a respreader 94-3, and a combiner 95.
The despread processing parts 91 and the spread processing parts 94 are respectively provided to received delayed waves, that is, multipaths. The structure shown in FIG. 2 is configured so as to handle three paths. In FIG. 2, signals corresponding to the respective paths are given a suffix xe2x80x9cixe2x80x9d (In FIG. 2, i=1-3). The signals corresponding to the paths are referred to RAKE fingers.
The despread processing part 91 is supplied with the error signal ej-1 of the previous stage and the interference replica signals Sj-1,1-Sj-1,k (these signals of the first stage are zeros). The despreader 91-1 receives the error signal ej-1 from the previous stage (the received signal R0 in the first stage) and performs a despread operation thereon using the spread code. A suffice xe2x80x9cjxe2x80x9d indicates the stage identification number.
The adder 91-2 adds the despread signal and the interference replica signals Sj-1,2-Sj-1,k (which are zeros in the first stage), and creates a resultant receive symbol R1 of the first path. The channel estimation circuit 91-4 receives the receive symbol R1, and estimates channels of paths (the characteristics of transmission paths) using pilot symbols shown in FIG. 3B. Thus, channel estimate values "xgr"i{circumflex over ( )} are obtained for the respective paths.
The despread signal Ri is multiplied by a complex number "xgr"i{circumflex over ( )} * of the channel estimate "xgr"i{circumflex over ( )} by the multiplier. Hence, a received symbol can be obtained from which a phase error due to influence of the transmission paths has been eliminated.
The output signals of the multipliers 91-3 related to the respective paths are diversity-combined (maximal ratio combining) by the combiner 92. A resulting receive symbol xcexa3Ri "xgr"i{circumflex over ( )} * obtained by the maximal ratio combining is compared with the decision part 93, so that a data symbol can provisionally be decided.
The signals generated and output by the respread processing parts 91 are called interference replica generation signals. The interference replica generation signals are converted into interference replica signals and interference residual signals, which are then transferred to the next stage.
The provisionally decided symbol Zs{circumflex over ( )} output by the decision part 93 branches into signals corresponding to the paths. In each of the spread processing parts 94, the multiplier 94-1 multiplies the provisionally decided symbol Zs{circumflex over ( )} by the channel estimation value "xgr"i{circumflex over ( )}. Hence, the provisionally decided data symbol is decomposed into the signals corresponding to the respective paths, which are output to the next stage as interference replica signals Sj,1-Sj,k.
The adders 94-2 of the spread processing parts 94 respectively add the interference replica signals Sj,i-Sj,k that are output by the multipliers 94-1 and correspond to the paths and the interference replica signals Sj-1,1-Sj-1,k supplied from the previous stage. Then, the adders 94-2 respectively output the differences between the interference replica signals Sj,i-Sj,k of this stage and the interference replica signals Sj-1,i-Sj-1,k. The output signals of the adders 94-2 of the spread processing parts 94 are spread using a spread code in the respective respreaders 94-3. The respread output signals of the respreaders 94-3 corresponding to the respective paths are combined by the combiner 95. The output signals of the combiners 95 of the interference canceller units provided for the respective users"" channels are output to the combiner 82 shown in FIG. 1 as interference residual signals dj,1-dj,k.
FIG. 3A shows a conventional final-stage receiver provided in the final stage of the multistage type interference canceller, and FIG. 3B shows a frame format. The final-stage receiver labeled 100 in FIG. 3A includes despread processing parts 101, a combiner 102 and a decoder 103.
The despread processing parts 101 of the final-stage receiver 100 are supplied with the error signal em-1 from the interference replica generation unit of the previous stage and the interference replica signals Sm-1,1-Sm-1,k, and perform the same process as that of the aforementioned despread processing parts 91 of the interference canceller unit. Hence, received symbols can be obtained.
Each of the despread processing parts 101 of the final-stage receiver 100 is equipped with a despreader 91-1, an adder 91-2, a multiplier 91-3, and a channel estimation circuit 91-4, which are the same as corresponding those of the despread processing part 91 of the interference canceller unit.
The combiner 102 of the final-stage receiver 100 performs diversity combining (maximal ratio combining) of the received symbols output from the despread processing parts 101. The resultant receive symbol xcexa3Ri "xgr"i{circumflex over ( )} * obtained by the maximal ratio combining is compared with a threshold value by the decoder 103. Hence, a data symbol can be reproduced.
Referring to FIG. 3B, a pilot symbol 104 is interposed between information symbols 105, and is repeatedly transmitted by a transmitter so that it is located in a given time position. The pilot signal 104 is predetermined known data symbol, and the receive symbol received can be expressed as Zxc2x7"xgr" where Z denotes a value (complex number) of the pilot symbol 104.
Since the value of the pilot symbol 104 is known, the channel estimate circuit 91-4 multiplies the receive symbol Zxc2x7"xgr" by the complex conjugate Z* of the value Z of the pilot symbol, and thus outputs |Z|2xc2x7"xgr". Since the magnitude (amplitude) of the pilot symbol is known (may be equal to 1: |Z|=1), an estimate value of the transmission path characteristic "xgr" of the path. The aforementioned channel estimate circuit 91-4 averages the estimated transmission path characteristics "xgr" obtained using a plurality of pilot symbols. The average value "xgr" thus obtained is output as the channel estimate value.
FIG. 4 shows a receiver of a base station including the interference canceller. A signal received via an antenna (ANT) 110 is input to a radio part (Rx) 120, which then amplifies the received signal by means of an amplifier (LNA) 121. The amplified signal is applied to a band-pass filter (BPF) 122, which eliminates components located outside of a given band. A mixer 123 multiplies the output signal of the band-pass filter 122 by a local oscillation signal from a local oscillator LO. Thus, the received signal is converted into a signal in the base band. High-frequency components contained in the base-band signal are eliminated by a low-pass filter (LPF) 124. The output signal of the low-pass filter 124 is then output to the next stage.
An A/D converter 130 of the next stage samples the received signal from the radio part 120, and outputs a corresponding digital signal, which is applied to a path search circuit 140. The path search circuit 140 calculates delay times of the respective paths by using a plurality of delay waves received, and outputs delay time information obtained for the respective paths to an interference canceller 150.
The interference canceller 150 performs despreading for the respective paths in the interference replica generation units and the final-stage receivers on the basis of the delay time information obtained for the respective paths. Receive symbols thus obtained are output to decoders 160. Interference between the user channels (spreading codes) and interference between the paths have been eliminated from the receive symbols applied to the decoders.
Each of the decoders 160 compares the corresponding receive symbol from the interference canceller 150 with a threshold value. Thus, a data symbol can be decoded. Each of the decoders 160 shown in FIG. 4 is the same as the decoder 103 of the final-stage receiver shown in FIG. 3.
The interference canceller of the above-mentioned type, in which interference is eliminated by subtracting the interference replica signals of the users"" channels from the original multiplexed receive signal, has a disadvantage in that the interference canceling performance greatly depends on the precision of the interference replica signals generated as described above.
If the interference replica signals having a poor precision are subtracted from the receive signal, interference power may be increased and the receive performance may thus be degraded. The precision of the interference replica signals may be degraded and the interference eliminating capability may be degraded if the signal is received at a relatively low level, or an excessive large number of user channels with respect to the spreading ratio is used or if the transmission paths are quickly varied (under high-speed fading environment).
An object of the present invention is to provide an interference canceller in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide an interference canceller in which interference is eliminated taking into account the receive states of code-multiplexed signals of users"" channels so that improved transmission quality can be obtained.
The above objects of the present invention are achieved by an interference canceller comprising: despread processing parts; a combiner combining interference replica generation signals; a decision part that decides an output signal; spread processing parts coupled to the despread processing parts and the decision part; an attenuation coefficient generator generating an attenuation coefficient dependent on a reliability of the interference replica generation signals; and a multiplier multiplying the output signal of the decision part by the attenuation coefficient.
The interference canceller may be configured so that the attenuation coefficient generator generates the attenuation coefficient which has a value dependent on a position of said interference canceller in a multi-stage formation.
The interference canceller may be configured so that the attenuation coefficient generator generates the attenuation coefficient which has a value dependent on a delay of time of a path through which a signal applied to the interference canceller is propagated.
The interference canceller may be configured so that the attenuation coefficient generator generates the attenuation coefficient which has a value dependent on a number of channels multiplexed.
The interference canceller may be configured so that the attenuation coefficient generator generates the attenuation coefficient which has a value dependent on a level of a signal applied to the interference canceller.
The interference canceller may be configured so that the attenuation coefficient generator generates the attenuation coefficient which has a value dependent on a level of a signal received through an antenna branch.
The interference canceller may be configured so that the attenuation coefficient generator generates the attenuation coefficient which has a value dependent on a level of a signal propagated through a path and applied to the interference canceller.
The interference canceller may be configured so that the attenuation coefficient generator generates the attenuation coefficient which has a value dependent on a ratio of signal power to interference/noise power.
The interference canceller may be configured so that the attenuation coefficient generator generates the attenuation coefficient which has a value dependent on at least two factors indicating a state of receiving a signal.
The interference canceller may be configured so that the attenuation coefficient generator generates the attenuation coefficient which has a value which is increased as the reliability of the interference replica generation signal is degraded.
The interference canceller may be configured so that: the interface canceller includes a plurality of stages; and each of the stages includes the despread processing parts, the combiner, the decision part, the spread processing parts, the attenuation coefficient generator and the multiplier.